The present invention relates to a micro power converter miniaturized by integrating a micro magnetic element and a semiconductor chip, in particular to a micro power converter of a step-up and step-down type. A micro power converter in power converters represented by DC-DC converters (that are switching power supplies) is generally composed by integrating a micro magnetic element and a semiconductor chip to miniaturize the converter. FIGS. 6(a), 6(b), and 6(c) show an example of construction of a micro inductor used in a micro power converter. The inductor is composed of a conductor 2 wound on and through a planar magnetic core 1, forming a solenoid coil, as shown in FIGS. 6(a), 6(b), and 6(c). FIG. 6(a) is a plan view of the inductor 10 provided with the conductor 2 and terminals 3 on the magnetic core 1; FIG. 6(b) is a sectional view of the micro inductor cut along the line a-a′ in FIG. 6(a) showing an arrangement of the conductor 2 on the magnetic core 1 and connection of the conductor 2 on the both surfaces of the magnetic core 1; and FIG. 6(c) is a sectional view of the micro inductor cut along the line b-b′ in FIG. 6(a) showing an arrangement of the conductor 3 on the magnetic core 1 and connection of the terminals 3 on both surfaces of the magnetic core 1.
FIG. 7 and FIG. 8 show examples of construction of micro power converters using the micro inductor 10 shown in FIG. 6. The converter of FIG. 7 has a structure in which terminals 3 are connected to pads 5 provided on a front surface 21 of a semiconductor chip 20 through joints 4 (that are bumps), connecting the micro inductor 10 and the semiconductor chip 20. The converter of FIG. 8 has a structure in which terminals 3 are connected to pads 5 provided on a front surface 21 of a semiconductor chip 20 through wires 6, connecting the micro inductor 10 and the semiconductor chip 20. The structures of FIG. 7 and FIG. 8 are different from each other in that the back surface 22 of the semiconductor chip 20 faces outside with respect to the micro inductor 10 in the structure of FIG. 7 and inside in the structure of FIG. 8.
Japanese Unexamined Patent Application Publication No. 2004-072815 discloses a basic structure of a micro power converter having a construction similar to that of the micro power converter shown in FIG. 7. Japanese Unexamined Patent Application Publication No. 2007-081146 discloses a basic structure of a micro power converter having a construction similar to that of the micro power converter shown in FIG. 8. Japanese Unexamined Patent Application Publication No. 2000-243630 listed below discloses a planar transformer constructed using through holes. The conventional micro power converters are miniaturized by integrating a micro inductor 10 and a semiconductor chip 20 in a structure as shown in FIG. 7 and FIG. 8.
FIG. 9, FIG. 10, and FIG. 11 show examples of actual circuits to implement a micro power converter. The circuit example of FIG. 9 is the one for implementing a step-down type converter; the circuit example of FIG. 10 is the one for implementing a step-up type converter; and the example of FIG. 11 is the one for implementing an inverting converter. Every example in the three Figures constructs a micro power converter 30 with a micro inductor 10 and a semiconductor chip 20 incorporating two or one of the semiconductor switches S1 and S2. No circuit example of FIG. 9, FIG. 10, and FIG. 11 is an example of a step-up and step-down type converter.
The circuits shown in FIG. 9, FIG. 10, and FIG. 11 may be combined to arrive at the circuit shown in FIG. 12. However, the resulting circuit is deficient for at least the following reasons. In the circuit construction shown in FIG. 12, four semiconductor switches must be incorporated. Here, the on-resistance of switches in a step-down operation is the sum of the resistances of two series-connected switches S1 and S4 or S2 and S4, and on-resistance of switches in the case of step-up operation is the sum of resistances of two series-connected switches S1 and S3 or S1 and S4. Because circuit resistances increase due to the increased resistance of switches, efficiency is degraded. In order to counteract this degradation of efficiency, semiconductor switches S1 through S4 must be enlarged in order to alleviate the degradation of efficiency. Power consumption in semiconductor chip 20′ increases in order to drive the four enlarged switches S1 through S4, and, therefore, the semiconductor chip 20′ increases in size due to the above-mentioned issues.
Japanese Unexamined Patent Application Publication No. H07 177740 listed below discloses use of a flyback converter for a step-up and step-down means. FIG. 13 shows an example of an ordinary circuit for implementing a flyback converter. Switch S2 in FIG. 13 can be replaced by a diode, likewise implementing the converter.
A conventional transformer is larger than other components of a converter and, as a matter of course, is larger than the inductor 10 used in the step-up and step-down converter shown in FIG. 12. In practice, the size of a transformer is a primary factor in determining the size of a power supply. Therefore, there is no teaching in the common practice by those skilled in the art to use a flyback converter to miniaturize a step-up and step-down converter of FIG. 12; that is, there is no teaching for using a flyback converter for the purpose of implementing a micro power converter of a step-up and step-down type. Rather, the usage of a flyback converter conventionally increases the size of a power converter large-sized. As described above, semiconductor switches are conventionally designed to be increased in number and size in order to improve efficiency. The large size of a semiconductor chip due to the conventional design has not been addressed because an inductor and a transformer are typically much larger than a semiconductor chip and thus, the size of a semiconductor chip itself affects the size of an overall power supply very little.
For implementing a conventional step-up and step-down converter in a micro power converter in these circumstances, the number of semiconductor switches remains at four and the size of a semiconductor chip is reduced by devising layout of the semiconductor chip and semiconductor production process. relying only on these methods of size reductions inhibits implementation of a micro power converter.